1. Field of the Invention
The invention relates to a laser system resonator optics monitoring method, and particularly to on-line monitoring of the quality of optical components of an installed line-narrowing module.
2. Discussion of the Related Art
Semiconductor manufacturers are currently using deep ultraviolet (DUV) lithography tools based on KrF-excimer laser systems operating around 248 nm, as well as the following generation of ArF-excimer laser systems operating around 193 nm. The ArF and KrF lasers have a broad characteristic bandwidth typically around 300-400 pm (FWHM). Vacuum UV (VUV) uses the F2-laser which characteristically emits two or three closely spaced lines around 157 nm.
Integrated circuit device technology has entered the sub quarter micron regime, thus necessitating very fine photolithographic techniques. The minimum size of a structure that may be produced on a silicon wafer is limited by the ability to optically resolve the structure. This resolution ability depends directly upon the photolithographical source radiation and optics used.
It is important for their respective applications to the field of sub-quarter micron silicon processing, that each of the above laser systems become capable of emitting a known narrow spectral bandwidth around a precisely determined and finely adjustable absolute wavelength. Techniques for reducing bandwidths by special resonator designs to less than 100 pm (for ArF and KrF lasers) for use with all-reflective optical imaging systems, and for catadioptric imaging systems to less than 0.6 pm, are being continuously improved upon.
A line-narrowed excimer or molecular fluorine laser used for microlithography provides an output beam with specified narrow spectral bandwidth. It is desired that parameters of this output beam such as wavelength, bandwidth, and energy and energy dose stabilty be reliable and consistent. Narrowing of the bandwidth is generally achieved through the use of a bandwidth narrowing and/or wavelength selection and wavelength tuning module (hereinafter xe2x80x9cline-narrowing modulexe2x80x9d) including most commonly prisms, diffraction gratings and, in some cases, optical etalons and mirrors.
A beam expander of the line-narrowing module typically functions to match the beam to the dimensions of the dispersion element to optimize its dispersive power. In most cases, the beam expander includes one or more beam expanding prisms. The dispersive element of the line-narrowing module typically functions to disperse the incoming beam after magnification by the beam expander. The light is typically dispersed angularly such that light rays of the beam with different wavelengths are reflected at different angles from the dispersion element. Only those rays fitting into a certain xe2x80x9cacceptancexe2x80x9d angle of the resonator undergo further amplification, and eventually contribute to the output of the laser system. Besides narrowing the bandwidth, then, one or more components of the line-narrowing module can typically be rotated for tuning the output emission wavelength of the laser system, and other tuning methods are being developed (see, e.g., U.S. patent application Ser. No. 60/178,445, which is assigned to the same assignee as the present application and is hereby incorporated by reference).
Line-narrowing modules vary in their response to the exposure to high power or high repetition rate laser beams that cause heating and aging of the optical components. For example, nonuniform heating of the optical elements of the line-narrowing module may substantially degrade their quality by, for example, disrupting the planarity of optical surfaces by localized expansion and causing fluctuations in the thermally dependent refractive index, thereby distorting the wavefront of the retroreflected or transmitted beam and detuning the wavelength. Wavefront distortions lead to changes in the output bandwidth of the laser system which is a parameter that it is desired to keep constant. Wavelength detuning may be compensated by rotating an optical element, typically the grating or HR mirror, as mentioned above. In addition to wavefront distortions and detuning, absorption by optical components results in reduced overall efficiency of the laser.
Parameters of the line-narrowing module that depend on the xe2x80x9cqualityxe2x80x9d of the optical components such as the magnitude of angular dispersion, reflectivity for specific wavelengths, linearity (i.e., absence of wavefront distortions), scattering of the beam. etc., will thus affect the bandwidth, linewidth and overall performance of the laser. The quality of the optical components may be generally tested by measuring the absorption of DUV (or VUV for F2 lasers) radiation that leads to hearing of the optical components and thermally induced distortions and defects, such as by measuring an intensity reduction by an incoming and outgoing beams through at material or optical component, or by measuring fluorescence emitted from materials being impinged upon by a DUV or VUV laser beam.
There are techniques for testing the quality of optical components of a line-narrowing module both individually and collectively prior to installation into a laser resonator (see U.S. patent application Ser. Nos. 09/454,803 and 60/178,804, which are assigned to the same assignee as the present application, and U.S. Pat. No. 5,894,392, each of which is hereby incorporated by reference). The ""803 application discloses to test a line-narrowing module, or components thereof, by directing a beam of known wavelength through the test module or components and measuring parameters such as the spatial profile and bandwidth of the reflected beam. The ""804 application discloses to test characteristics of optical materials such as absorption, such as by measuring fluorescence or intensity reduction of incident beams as they traverse the material, and homogeneity prior to forming optical components for use in line-narrowing applications. The ""392 patent discloses to contact optical prisms with temperature sensors prior to assembling them into a line-narrowing module. By measuring the rate of temperature increase, the absorption of each prism and its quality is determined. It is desired to have a technique for quality monitoring or testing of optical components of a line-narrowing module on-line and moreover without installing intrusive thermal sensing devices into the laser resonator.
It is therefore an object of the invention to provide a technique for online monitoring or testing of the quality of optical components of a line-narrowing module of a laser system.
In accord with this object, a technique is provided wherein the output wavelength of a line-narrowed laser system drifts from a desired value as one or more optical components of the line-narrowing module is heated due to absorption of radiation of an operating laser system. The wavelength is tuned back to the desired wavelength using a feedback loop by rotating or otherwise adjusting one or more optical components of the line-narrowing module. The amount of rotation or adjustment of these optical components is monitored for indirectly measuring, and thereby testing, the absorption or quality of the optical components.
The grating, one or more prisms and/or one or more etalons may be rotated, or the pressure within an enclosure of one or more optics of the line-narrowing narrowing module may be adjusted, to adjust the wavelength back to the desired value. Preferably, both the grating and an etalon (when an etalon is used) are adjusted together to maximize the efficiency of the resonator. One or both of these adjustments may be monitored for quality testing. The testing is preferably performed when the laser is operating in cw mode, rather than burst mode.